The invention relates to a magnetic resonance (MR) method in which a navigator pulse is generated so as to excite the nuclear magnetization in a spatially limited volume by means of at least one RF pulse and at least two gradient magnetic fields having gradients which extend differently in respect of time and space, after which at least one MR signal is received from the volume, in conjunction with a further gradient magnetic field, so as to be evaluated. The invention also relates to an MR apparatus for carrying out such a method as well as to a computer program for a control unit of an MR apparatus for carrying out such a method.
A method of the kind set forth is known from European patent application EP-A 0793113, as well as from an article by Nehrke, Bxc3x6rnert et al., in the magazine xe2x80x9cMagnetic Resonance Imagingxe2x80x9d, Vol. 17, No. 8, pp. 1173-1181, 1999. Navigator pulses enable the excitation of a zone limited in two dimensions, for example a cylindrical bar (pencil beam) . When such a navigator pulse is incident, for example on the diaphragm of a patient, the state of motion thereof (or the respiration of the patient) can be determined by evaluation of the MR signals received after the navigator pulse. These pulses can be used for the detection of the state of motion of an object to be examined, for example in the so-called gating techniques.
Because the strength of the gradient is limited, only a region of limited dimensions and of finite density can be traversed in the k space during the finite duration of the navigator pulse. This limits the resolution and also give rise to the excitation of undesirable aliasing structures which may obstruct the detection of the state of motion. If navigator pulses of longer duration were used so as to enable a larger region to be traversed in the k space with a larger density, so-called xe2x80x9coff-resonancexe2x80x9d effects would become manifest, for example due to the inhomogeneity of the steady magnetic field whereto the object to be examined is exposed during an MR examination. Because of such effects the region excited by the navigator pulse is extended so that the evaluation of the MR signals is also impeded.
It is therefore an object of the present invention to conceive a method of the kind set forth in such a manner that a more favorable compromise is reached between the need for an enhanced resolution, or reduced aliasing structures, on the one hand and the duration of the navigator pulses on the other hand.
This object is achieved in a first manner which is characterized in that the variation in time of the gradient magnetic fields is such that there are generated at least two MR signals which correspond to an excitation in the k space along a mutually offset trajectories, and in that the MR signals are combined.
The two (or more) MR signals can be generated by means of two (or more) RF pulses, the gradient fields that are active in conjunction therewith being configured in such a manner that the k space is traversed along offset trajectories. When these MR signals (or the Fourier transforms of these MR signals) are combined, the same spatial resolution, or reduction of the aliasing structures, can be achieved as if the k space were excited along all trajectories by means of a single RF pulse of the same duration.
However, such MR signals can also be generated by utilizing only a single RF pulse and by reading out the excited nuclear magnetization several times along different trajectories. This yields the same effect as would be obtained by using an RF pulse of the same duration and gradient fields having such a strength that all (mutually offset) trajectories were traversed with this single RF pulse.
The k space can then be traversed in various ways, for example along helical trajectories extending radially through the origin of k space. However, by implementing the inventive method herein such that a first gradient magnetic field (Gx) is generated during the RF pulse, its gradient extending in a first direction and its polarity being repeatedly reversed between a positive and a negative value, and a second gradient magnetic field (Gy1; Gy2), having a gradient extending in a second direction, is generated during the polarity reversal of the first gradient magnetic field (Gx), k space is traversed along parallel trajectories upon excitation by the RF pulse. This excitation is an analogy to the so-called RPI sequence in which the k space is sampled along parallel trajectories after an excitation.
In conjunction with such excitation of the k space, in conformity with claim 3 it is possible to acquire at least two MR signals after an RF pulse. These MR signals correspond to the MR signals that would be obtained after excitation by several RF pulses where the k space would be traversed along several, mutually offset groups of trajectories.
In a further embodiment, a third gradient magnetic field (Gz) is generated after the RF pulse, its gradient extending in a third direction and its polarity being reversed at least once between a positive and a negative value, where a gradient magnetic field is generated with a gradient extending in the second direction during the reversal of the polarity of the first field whose gradient extends in the second direction, the time integral over this gradient is smaller than that during the RF pulse, a respective MR signal is read out each time for a polarity of the third gradient magnetic field and the MR signals are combined. Such method provides that k space is traversed or sampled along parallel trajectories during the excitation as well as during the reading out. In these circumstances, however, the nuclear magnetization is not excited in a single xe2x80x9cpencil beamxe2x80x9d as desired but also, due to the aliasing effect, in further regions, (so-called aliasing peaks) which recur periodically in the second direction and affect the evaluation of the navigator signal.
But when such method is further qualified to assure that the polarity of the third gradient magnetic field is reversed once, and the time integral over the second gradient amounts to half the time integral over this gradient during a polarity reversal of the first gradient magnetic field , the effects of the two aliasing peaks to both sides of the region actually to be excited can be eliminated by combination of the two MR signals then acquired. And where the polarity of the third gradient magnetic field is reversed twice from one polarity to the other, and during such a polarity reversal, the time integral over the second gradient amounts to one quarter of the time integral over the second gradient during a polarity reversal of the first gradient magnetic field, the effect of two further aliasing peaks can additionally be compensated.
Whereas in the embodiments so described, several (two or four) MR signals are acquired by means of only a single RF pulse, only one MR signal is received for each RF pulse where a region is excited by a plurality of navigator pulses along mutually offset trajectories in k space, a respective MR signal is received, and the MR signals are combined. Such a method makes sense in the case of continuous generation of navigator pulses while the trajectories along which k space is traversed during the excitation change cyclically.
In a variation on the above-described methods, the invention includes an MR method in which a navigator pulse is generated so as to excite the nuclear magnetization in a spatially limited volume by means of at least one RF pulse and at least two gradient magnetic fields having gradients which extend differently with respect to time and space, after which at least one MR signal is received from the volume in conjunction with a further gradient magnetic field for evaluation. During the excitation, the gradient magnetic fields are generated with a variation in time such that the trajectories are limited to one half of the k space, the resultant MR signal (comprising a real part and an imaginary part) is received in a phase-sensitive manner, and the real part of the MR signals is exclusively evaluated.
The trajectories followed are therefore limited to one half of the k space. Moreover, the real part of the nuclear magnetization vector remains unmodified (in comparison with a method where both halves of the k space are traversed with the same density of the trajectories). It follows that the imaginary part leads to widening. When only the real part of the MR signal is used instead of the absolute value of the MR signal during the evaluation of the navigator signal, the same spatial resolution can be achieved as before. This method can be used for all trajectories which symmetrically excite the k space. In conformity with the claims appended hereto, however, it is particularly effective to traverse the k space along parallel trajectories during the excitation, analogous to the EPI sequence during the reading out of MR signals.
The first solution, in as far as it is limited to two MR signals, and the second solution utilize given effects (that the effect of the aliasing peaks can be eliminated by combination of the MR signals or that the real part of the nuclear magnetization excited in these peaks does not lead to widening).
MR apparatus for carrying out the MR methods described herein is also set forth in the detailed description to follow, and the claims appended hereto. The software for a control unit of an MR apparatus which is suitable for carrying out the aforementioned methods of this invention are also described in the following detailed description, and in the claims appended hereto.